FIG. 6 is a diagram of a boiling water reactor. To increase power density, the boiling water reactor employs a jet pump system that combines a recirculation pump, provided outside a reactor pressure vessel 1, and jet pumps 11, provided inside the reactor pressure vessel 1.
The reactor pressure vessel 1 accommodates a coolant 2 and a reactor core 3. The reactor core 3 is made up of a plurality of fuel assemblies and a plurality of control rods, and is surrounded by a core shroud 10. The coolant 2 flows through the reactor core 3 upward. While flowing through the reactor core 3, the coolant 2 is heated due to heat produced by nuclear reaction in the reactor core 3, generating two-phase flow of water and steam. The two-phase flow of the coolant 2 flows into a steam separator 4, provided above the reactor core 3, to be separated into water and steam.
The resulting steam is introduced into a steam dryer 5, provided above the steam separator 4, to turn to dry steam. The dry steam is transferred through a main steam line 6, connected to the reactor pressure vessel 1, to a steam turbine to be used for power generation. The resulting water flows down a downcomer 7, provided between the reactor core 3 and the reactor pressure vessel 1, into a space below the reactor core 3.
The control rods are inserted into or withdrawn from the reactor core 3 through control rod guide tubes 8, which are provided below the reactor core 3. A plurality of control rod drive mechanisms 9 are provided below the control rod guide tubes 8 to control the insertion and withdrawal of the control rods.
A plurality of jet pumps 11 are provided in the downcomer 7 at circumferentially equal spacings. The recirculation pump, not shown, is provided outside the reactor pressure vessel 1. The recirculation pump, the jet pumps 11, and a recirculation line routed between the recirculation pump and the jet pumps 11 constitute a recirculation system. The recirculation pump supplies driving water to the jet pumps 11, so that the jet pumps 11 operate to provide forced circulation of the coolant 2 into the reactor core 3.
FIG. 7 is a diagram of an arrangement of one of the jet pumps 11 in FIG. 6.
In FIG. 7, reference numeral 12 refers to a riser pipe of the jet pump 11. The riser pipe 12 is attached on the reactor pressure vessel 1. The riser pipe 12 is connected to a recirculation inlet nozzle 13 of the recirculation pump. The coolant 2 is supplied through the recirculation inlet nozzle 13 to the riser pipe 12 to be introduced into the reactor.
An upper portion of the riser pipe 12 is connected by a transition piece 14 to a pair of elbows 15A and 15B. The elbows 15A and 15B are connected through mixing nozzles 16A and 16B to inlet throats 17A and 17B, respectively. The inlet throats 17A and 17B are connected to diffusers 18A and 18B, respectively.
As the coolant 2 is jetted by the mixing nozzles 16A and 16B into the inlet throats 17A and 17B, the letting involves surrounding water, allowing the jets of the coolant 2 and the involved water to be mixed in the inlet throats 17A and 17B. The diffusers 18A and 18B then allow hydrostatic head to be restored.
To control power of a nuclear power plant with the reactor including the coolant recirculation system using the jet pumps 11 as described above, it is important to measure flow rates in the et pumps 11 during normal operation. For this purpose, flow rate measurement pipes 19 are provided on the diffusers 18A and 18B as illustrated in FIG. 7 to measure a static pressure difference between upper and lower portions of each of the diffusers 18 during the operation. The measurement is compared to a calibration value, which is a measurement taken before the operation of the plant, to calculate a flow rate of the jet pumps 11. The measurement pipes 19, which are welded to static pressure holes located at the upper and lower portions of the diffusers 18A and 18B, are supported by connecting members 24 fixed to the diffusers 18A and 18B.
As illustrated in FIGS. 8A and 8B, the measurement pipes 19 are arranged in an annular space 29 between the reactor pressure vessel 1 and the shroud 10. The measurement pipes 19 are routed in the annular space 29 in a complex manner to be connected through a jet pump measurement nozzle 27 to a pipe arrangement outside the reactor. Two such jet pump measurement nozzles 27 are symmetrically positioned in the reactor pressure vessel 1.
Because of flows of the coolant 2 fed from the recirculation pump, the jet pumps 11 as described above are used under more severe conditions than other devices. This results in significant load acting on components of the jet pumps 11. The measurement pipes 19, in particular, are affected by fluid oscillation around the diffusers 10 and other factors directly or via the connecting members 24, leading to a significant stress acting in the measurement pipes 19. The measurement pipes 19 thus may be susceptible to cracks and ruptures.
To reduce stress caused in the measurement pipes 19 due to the fluid oscillation and the like, various improvements have been traditionally made for the fixing of the measurement pipes 19.
In the event of a crack or a rupture caused in any of the measurement pipes 19 or its welded portion to a corresponding one of the connecting members 24 in the jet pump system in the reactor, it is necessary to conduct repair work from above the reactor core, which is located in a control area for radiation, in a remote manner. Additionally, this repair work involves underwater work.
It is desirable that the measurement pipes 19, when repaired, is fixed to a corresponding one of the diffusers 18A and 18B robustly so that stress to be applied to the welded portion to the connecting members 24 due to the fluid oscillation is reduced as much as possible. It is, however, extremely difficult to access the measurement pipes 19, which is located in the extremely narrow space, to fix it robustly from above the reactor core remotely.
As a solution to the issues described above, a reactor measurement-pipe maintenance clamp apparatus is provided which is capable of facilitating fixing a measurement pipes, routed in an extremely narrow space, robustly to the diffuser by remote control from above the reactor core and reducing stress in a welded portion of the measurement pipe to a support.